1. Field of the Invention
The present invention relates to a tungsten skeleton structure fabrication method employed in application of a copper infiltration and tungsten-copper composite material fabrication method thereof capable of preventing injection molded body from being unevenly infiltrated during a liquid phase sintering thereof.
2. Description of the Prior Art
A semiconductor device provided on a substrate and composing of a circuit produces heat when an electrical power is supplied thereto. The heat produced may damage functions or the integrity of the circuit, and thus a heat sink which serves to efficiently eliminate heat needs to be attached to the substrate, for thereby optimizing the circuit operation.
Materials for the heat sink must be provided with a high thermal conductivity so that heat cannot accumulate on attached interfaces, and further a thermal expansion coefficient of a heat sink material must be similar to that of the semiconductor substrate to avoid a thermal stress build-up.
When the heat sink is comprised of a tungsten-copper composite material, the respective functions of a GaAs FET and a GaAs MMIC for a microwave device can be improved, as disclosed in U.S. Pat. Nos. 4,942,076/4,988,386/5,563,101. This is because tungsten particles with low thermal expansion coefficient are evenly dispersed within a copper matrix having a high thermal conductivity, thus improving the thermal conductivity of the heat sink. Also, the thermal expansion coefficient of the tungsten-copper heat sink desires to correspond to the thermal expansion coefficient (6.7 ppm/.degree.C.) of a GaAs substrate.
However, tungsten W is a metallic element having a high melting point (3410.degree. C.) and a high density (19.3 g/cm.sup.3) but copper Cu, also a metallic element, but with a low melting point (1083.degree. C.) and a low density (8.96 g/cm.sup.3) relative to that of tungsten W, therefor it is difficult to fabricate respective composite materials having an evenly fine structure using the two metals in accordance with a general melting and forming method. In order to overcome such a difficulty, a powder metallurgy technique was employed as disclosed by N. M. Parikh and M. Humenik JR, J. Amer. Cer. Soc., Vol. 40 1957, pp. 315-320, and in Korea Patent Publication No. 96-15218.
A liquid phase sintering and a Cu-infiltrating method are employed to fabricate a tungsten-copper composite material in the powder metallurgy application. In the liquid phase sintering (as disclosed in B. Yang and R. M. German, Tungsten and refractory Metals-1994, eds. A, Mose and R. J. Dowding MPIF, Princeton, N.J., 1995, pp. 237-244), a tungsten powder and a copper powder are admixed and the admixture is sintered for several hours at a temperature ranging from 1150.about.1550.degree. C., thus the temperature range is higher than the melting point of copper. In the Cu-infiltrating method, first the powder tungsten is molded and sintered in a preliminary step, a tungsten skeleton structure is fabricated wherein the liquid copper is infiltrated by capillary force.
When it comes to a liquid phase sintering, a solid solubility of tungsten into liquid copper is less than 10.sup.-7 weight percent, which may be virtually ignored, at a temperature ranging from 1300.about.1400.degree. C. (V. N. Eremenko, R. V. Minakova and M. M. Churakov. (Poroshkovaya Metallurgiya, No. 4, 1977, pp. 55-58)/Seung-Ki Joo, Seok-Woon Lee and Tae-Hyoung Ihn, Metall. and Mater. Trans. A, vol. 25A, 1994, pp. 1575-1578)
The wettability of liquid copper against tungsten surface is poor, and it is difficult to completely remove all pores in the tungsten-copper sintered bodies. Therefore, trace amount of transition metals, such as Ni, Co and Fe are added thereto to improve wettability and sintered, as disclosed in U.S. Pat. No. 4,788,627/J. L. Johnson and R. M. German, Metall. Trans. A, vol. 24A, 1993, pp. 2369-2377/Seung-Ki Joo, Seok-Woon Lee and Tae Hyoung Ihn, Metall. and Mater. Trans. A, vol. 25A, 1994, pp. 1575-1578. The transition metals improve the wettability of copper, and yet form a solution with the copper. Also, an amount of the transition metals sufficient to form an intermetallic compound with the tungsten is added, so as to urge an inter-grain boundary diffusion of the tungsten, thus improving sinterability. However, a sudden shrinkage occurring during the liquid phase sintering makes it difficult to control its shaping when a sinter body has a complicated form, and at the same time the added transition metal deteriorates the thermal conductivity of the copper.
In the Cu-infiltration method for the fabrication of the tungsten-copper composite, the transition metals are not added as in the liquid phase sintering method; instead, a liquid copper is infiltrated by controlling the tungsten skeleton structure for thereby fabricating a Cu-network structure in which the copper is evenly distributed, whereby the thermal conductivity is improved relative to that of the tungsten-copper composite fabricated by the liquid phase sintering. In particular, when the above-described tungsten skeleton structure fabrication method employs the activation sintering of pure tungsten instead of a liquid phase sintering by adding transition metals, a sudden shrinkage does not occur during sintering, for thereby facilitating a molding and size control of the shaping body, as disclosed in Y. Kai, C. Yamasaki, K. Yukuhiro and T. Okabe, Tungsten and Refractory Metals-1994. eds. A. Bose and R. J. Dowding, MPIF, Princeton, N.J. 1955, pp. 253-258. However, in order to fabricate a W--Cu(10.about.20 wt. %) composite material having a high thermal conductivity and a thermal expansion coefficient similar to that of a GaAs substrate, the tungsten skeleton structure must be adjusted to have a porosity of 20.about.35% (=1-{shaped body density/19.3}!.times.100).
Also, in order to fabricate the preliminary tungsten sinterred body with such a porosity, the sintering must be carried out at a temperature of 1500.degree. C. for extended period of time. Accordingly, in order to obtain such a high sintering temperature, a heating furnace using heating elements of MoSi.sub.2, tungsten W, or graphite, is required. Here, a high sintering temperature denotes a high energy input which causes the cost of the fabrication to increase.